1. Field of the Invention
The present invention relates to non-aqueous electrolyte secondary batteries comprising a positive electrode, a negative electrode, and a non-aqueous electrolyte.
2. Description of the Background Art
Non-aqueous electrolyte secondary batteries are commonly available today as secondary batteries having high energy density. In a non-aqueous electrolyte secondary battery, for example, charge and discharge occur by the transfer of lithium ions between a positive electrode and a negative electrode.
In such a non-aqueous electrolyte secondary battery, in general, a complex oxide of lithium transition metals having a layered structure of lithium nickelate (LiNiO2), lithium cobaltate (LiCoO2) or the like is used as the positive electrode, and a carbon material that can store and release lithium, a lithium metal, a lithium alloy, or the like is used as the negative electrode (refer to, for example, JP-2003-151549-A).
A specific discharge capacity as large as 150 to 180 mAh/g, a potential as high as approximately 4 V and a theoretical capacity as large as approximately 260 mAh/g can be obtained by using the above-mentioned non-aqueous electrolyte secondary battery.
In addition, an organic solvent such as ethylene carbonate or diethyl carbonate in which an electrolyte salt such as lithium borate tetrafluoride (LiBF4) or lithium phosphate hexafluoride (LiPF6) is dissolved is used as the non-aqueous electrolyte.
While these non-aqueous electrolyte secondary batteries have recently been used as power sources for mobile equipment, a need exists for developing non-aqueous electrolyte secondary batteries having higher energy densities with increasing power consumption caused by expansion in functionality of the mobile equipment.
With respect to lithium cobaltate (Li1-xCoO2) used presently as a positive electrode of a non-aqueous electrolyte secondary battery, if lithium ions of not less than 0.5 (=x) are released, a crystal structure collapses and reversibility (storage property and releasing property) is decreased. As a result, the specific discharge capacity that could be obtained is approximately 160 mAh/g at most.
In contrast, with respect to lithium nickelate (Li1-yNiO2) having the same crystal structure as that of LiCoO2, since lithium ions of up to approximately 0.7 (=y) can be released, the specific discharge capacity as large as approximately 200 mAh/g that is larger than the specific discharge capacity of LiCoO2 can be obtained.
However, as lithium ions are released, the crystal structure (crystal system) of the above-mentioned lithium nickelate changes to a hexagonal system, a monoclinic system, and a hexagonal system in this order. This change gradually makes the crystal structure of lithium nickelate collapse, resulting in a reduction in the reversibility similarly to lithium cobaltate.
Furthermore, nickel is unstable in the trivalent state (Ni3+) and liable to be in the divalent state (Ni2+). Therefore, lithium easily evaporates in a synthesis reaction at a high temperature and it is difficult to obtain a positive electrode active material having a stoichiometry.
In addition, since the radius of an ion of lithium is approximate to that of nickel, nickel is mixed in a lithium layer in many cases. As a result, diffusion of lithium is inhibited and it is difficult to produce the synthesis reaction.
In order to solve these problems, sufficient oxidation of nickel has been considered. For example, a method in which burning is carried out in oxygen and a method in which elements at nickel sites are replaced by a variety of elements to stabilize the crystal structure have been considered (refer to, for example, JP-2000-133249-A and H. Arai et al., J. Electrochem. Soc., 140, 1862 (1993)).
However, even if the above-described methods are used, it is difficult to sufficiently prevent diffusion of lithium from being inhibited and a synthesis reaction does not sufficiently occur.